1. Field of the Invention
This invention relates to the art of optical printing, and more particularly to improved means for controlling the exposure and/or color balance of the exposed copy over a considerably broadened exposure and color balance range. The invention is particularly useful when the copying rate is high enough to warrant full or partial pre-specification and/or automation of the exposure control and color correction processes.
2. Description of the Prior Art
It is commonly known in the photographic arts to make a photographic reproduction by projecting onto a light sensitive emulsion a focused image of a photographic master negative or positive, contained on a suitable transparent glass or plastic transparent base. The "focused projection" may be accomplished either through a suitable optical system, as is well known in the optical art, or by creating intimate contact between the master image bearing material and the sensitized emulsion or the copy film during the exposure time span. The sensitized emulsion may be fixed on some transparent material such as glass or clear plastic, as is the case for slide and motion picture duplication, or it may be fixed on some opaque medium such as paper as is the case for print production. In either case, some means is necessary for regulating the "exposure" (which may be broadly defined as ##EQU1## WHEREIN T IS TIME AND I(t) is the intensity of the illumination passing through the master image and striking the emulsion as a function of time) to insure that as much as possible of the detail present in the master image is reproduced in the copy.
Similarly, with color duplicators, the relative color balance of the exposing light source must be closely regulated to properly adjust for color balance aberations in the master image. In practice, due to the nature of the color photochemical process, the color balance may be adequately controlled by simultaneously controlling the relative exposure delivered by several discrete or partially overlapping regions of the color spectrum, which regions are usually selected to correspond to the peak sensitivites of the chemicals in the sensitized emulsion. These regions are usually red, green and blue for additive printers, and cyan, magenta, and yellow subtractive printers.
In practice, since the overall exposure variation is perceived as a separate parameter from color balance variations, it is convenient, though not always economically or physically practical, to control the overall exposure separately and independently from the ratios of exposures in the different color bands.
Throughout the following description it should be understood that it is often desirable to vary the color balance or exposure widely from the so-called "correct" values to achieve artistic effects or for other purposes. For example, in the art of motion picture duplication, it is common practice to vary the relative frame-by-frame exposure during some fixed number of frames from normal exposure to zero exposure or vice-versa. The resulting image density variation is known as a "fade" and is widely used for artistic effect.
In low speed copying devices such as photographic enlargers and the like, exposure is usually regulated by varying exposure time. The light source may be energized for a given time period or it may be maintained at a constant value and "chopped" by a mechanical shutter. Color balance is controlled by inserting different color filters or combinations in the light path, which selectively absorb some light from certain color spectral bands.
However, in high speed copiers, such as motion picture copiers, these techniques are not satisfactory. Normal tungsten light sources cannot maintain acceptable color stability if they are rapidly switched on and off, and the considerable thermal inertia of a metal filament virtually eliminates such switching as a viable exposure control means at the duplication speeds currently practised. Shutters, by reason of their slow speeds and lack of automation potential, are not used. The usual approach is to maintain the light source at a constant intensity and vary exposure by modulating the intensity of the light output beam. To this end numerous designs for electro-mechanical light modulating valves are in common use. The disadvantages of their use will be discussed below.
Since the usual motion picture master image roll contains numerous scenes spliced together, which may have been photographed under widely varying conditions of illumination, it is usually necessary to make many color balance corrections in the course of a single copy. Since changing filters would require the inconvenience of temporarily stopping the printing operation, another system is commonly used. The light beam from the source is physically split into several preferrably non-overlapping color bands, usually the primary colors, red, green and blue, by the action of suitable filters or dichroic mirrors as is well known in the art. Each beam is modulated separately with a separate electro-mechanical light valve, and the several beams are then recombined before being applied to the sensitized emulsion. This gives white light of variable color balance. When this approach is used then over-all exposure is usually regulated by operating the several color control valves in synchronization rather than with a fourth valve since the valves are expensive and transmit only part of the light that strikes them.
Perhaps the most obvious disadvantage in the use of electro-mechanical light valves for exposure and color control modulation is the sacrifice in speed and system versatility. The devices, because they incorporate moving parts, are fairly slow in response time as compared to the frame rates presently used in optical printers. Further, they offer a relatively small number of discrete intensity increments, usually only forty, for exposure and/or color balance variations.
It is correspondingly an object of the present invention to provide a high speed printing apparatus capable of implementing exposure and color corrections virtually instantaneously. A further object is to provide a printer capable of providing continuously adjustable color balance and exposure modulation over a wide dynamic range.
A further disadvantage of electro-mechanical light valves is their settling time. That is, after a command for a change in exposure is delivered, a certain period of time must elapse before the next command can be given.
It is correspondingly an object of the present invention to provide a high speed printing apparatus capable of implementing exposure and color correction commands fast enough so that a new command can be delivered on each frame if desired, even at very fast frame rates.
Because of the versatility limitations mentioned above, electro-mechanical light valves of the type used for exposure corrections cannot be used to create fades, which require smooth yet rapid variation over a large dynamic range. Thus a fourth light valve of a different construction is used to create fades. But even specially constructed fader valves usually offer a very limited number of fade lengths, and usually only the logarithmic intensity function can be implemented. In addition, the use of a fourth light valve adds to the complexity and cost of the machine.
It is correspondingly an object of the present invention to provide a high speed printer capable of implementing all exposure and fade and color balance corrections with only three light valves. A further object of the invention is to provide a printer on which color balance and exposure correction function can be controlled as completely independent orthogonal signals. It is also an object of the present invention to provide a printing apparatus with which fades of any length and conceivable single-valued mathematical function can be implemented, and with which the operator may specify any desired exposure or color balance value from a broad dynamic range for any frame.
The operation of electromechanical light valves can be pre-specified by placing strings of commands on paper tape or a similar digital recording medium. But due to the slowness of response of these modulators, it is impossible at currently practised printing frame rates to automate the color correction and exposure correction processes. Though it is a simple matter to automatically measure the density and color content of a given frame at a high speed, it is impossible with current electro-mechanical devices to make automatic compensation in exposure and color balance at the correct instant when the given frame has entered the printing aperture. This limitation necessitates a technician who visually examines the print in advance and assesses the needed exposure corrections and color balance by eye. He must then prepare a paper tape with the desired corrections to be fed as commands to the light valve system during the printing run.
It is correspondingly an object of the present invention to provide a printer which has the potential, by virtue of its instantaneous response characteristics, for complete automation of the exposure correction and color correction functions. Thus the machine operator must provide data ONLY for those frames for which a deviation from normal color correction or exposure correction is desired.
A further disadvantage of electro-mechanical light valves brought about by their complex mechanical structure is that the interface between the light valve and the control circuits is complex. This necessitates special codes for generating control commands. The codes are difficult to use, being designed for ease of device construction and not for user convenience.
It is correspondingly an object of the present invention to provide a printer capable of interfacing with control electronics capable of accepting color balance, exposure correction, and fade information commands in a format completely compatible with photographic terminology, and further, to provide versatile interface-ability with electronics capable of providing variability in the input language without the necessity for rewiring or re-designing the control circuitry.
While electro-mechanical light modulators can be designed to deliver reasonable absolute accuracy, they cannot be built with fast enough response times to incorporate them in servo-mechanisms operative at frequencies adequate for current printing frame rates. Such servo-mechanisms would allow light output regulation to be independent of light source intensity variations due to line ripple, power droop, or aging of the light source bulb. This deficiency results in the necessity of frequent, usually daily, test runs of the exposure and color balance of the light source. These test runs are both time consuming and costly.
It is correspondingly an object of the present invention to provide a printer which does not have to be calibrated, once initially adjusted. The calibration is entirely independent of long or short term variations in the intensity or color balance of the light source, whatever their causes. It is the invention's further object to provide a printer whose light source power supply does not have to be heavily regulated.
The aforementioned slowness of electro-mechanical light valve response has made it necessary for the motion picture industry to adapt a secondary device as a light chopper. This device is the familiar black leader spliced between shots during checkerboard editing, in a way well known in the motion picture art. The practice is so common that it is no longer generally realized that its necessity partially arises from a deficiency in present printer technology.
It is correspondingly an object of the present invention to provide a printer capable of rapid enough light modulation to wholly eliminate the necessity for opaque leader in checkerboard editing.
The inherent mechanical limitations of fading and exposure control valves makes it difficult or impossible to adjust their transfer functions to closely match the widely varying characteristc curves ("OD vs. log E curves") of different photographic emulsions. As a result, in some cases, fade and exposure corrections, however highly they match certain standardized curves, do not yield optical density corrections or functions in the print which conform closely to those curves. Thus a perfectly logarithmically exposed fade may not appear very logarithmic at all when the print is processed and screened. Another related deficiency often observed is a variation in color balance during fading.
It is correspondingly an object of the present invention to provide a printer whose fade, exposure, and color balance mechanisms can be easily interfaced with standard electronic calibration circuits exactly matched to the characteristic curves of a given film emulsion, which circuitry can be easily replaced or modified if the film stock is replaced.